Part of infant Earth survived moon's shocking birth

A BIG piece of early Earth may have resisted the apocalyptic melting caused by the impact that spawned the moon.

The leading idea for lunar origins is that a primordial planet named Theia slammed into Earth, scattering debris that congealed into the moon. Models suggested that the collision totally melted our planet, creating a ball of magma. In that scenario, convection should have mixed up the molten rock. After the planet cooled and the mantle reformed beneath the crust, it should have been left with the same isotope signatures throughout.

Sujoy Mukhopadhyay at Harvard University and his team analysed samples of mantle taken from sites around the globe. They found that the ratio of helium-3 to neon-22 is much higher in the shallow part of the mantle than in the deeper part. They also saw differences in the ratio of xenon-129 to xenon-130 in the layers.

"We think that the observations we have made provide some of the cleanest evidence that Earth didn't completely melt," says Mukhopadhyay. He described the results at the Goldschmidt 2014 conference in Sacramento, California, this week.

The team argues that the energy of the impact was not evenly distributed, allowing a rocky part of the planet far from the collision to survive. Over time, that rock spread out and formed its own layer within the mantle.

Oxygen-isotope measurements from a portion of Earth that survived the impact could reveal more about what our planet was like in its lost youth. That in turn may help us infer details about Theia, such as its size and composition.

This article appeared in print under the headline "Did part of 'lost' Earth survive moon's birth?"

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